CPM 440V (yet another Q): WHY so brittle?

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storyville

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Aug 11, 1999
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I have reviewed prior threads on CPM-440V via the search engine, and nearly all echo the caveat that it is fairly brittle. I could not find any comments to the contrary. I have also seen the bar graphs (on Cliff Stamp’s site -- thanks Cliff) indicating its relative wear resistance and toughness. Equally, however, I could find NO EXPLANATION of 440V’s apparent brittleness.

Yet Crucible’s own site, www.crucible.com, lists two benefits “For the End User” which suggest otherwise:

-- “Upgrade for Better Wear Resistance without Sacrificing Toughness (vs. Conventional Grades)”
-- “Improved Toughness (Less Chipping)”

I have read Crucible’s description of its “particle metallurgy” process and am wondering: why wouldn’t its resulting “100 density” and uniformity IMPROVE toughness along with wear resistance? Is it the “residual porosity” which the CPM process dramatically minimizes or eliminates which, ironically, promotes toughness?

More importantly, can any of you share EXPERIENCES confirming or challenging the notion of CPM 440V’s apparent brittleness? Virtually all of the references I’ve read in prior threads refer not to evidence gained from “real world use” but, rather, to its expected brittleness as noted “on paper.” (There are plenty of testimonies of its real world edge-holding, however...)

Particularly in a smaller (say 4” +/-) fixed blade, has 440V treated to above the established 56-58 RC posed any actual (as opposed to hypothetical) problems with chipping, or other evidence of brittleness?

In a variation of the APOLOGY genre now flowering on the forum, I apologize in advance (!) for my techno-stupid questions. I would have posted it to the General Forum, but this kind of question there increasingly appears to attract wide-ranging, bizarre replies from folks who admit to having no experience or other basis for their comments (e.g., “I heard that 440V chipped and rolled when someone punched it through a tank,” etc.). I’m considering ordering a 440V blade in the new year (millenium!), and would like to make as informed a decision as possible.

Thanks and regards,
Glen

[This message has been edited by storyville (edited 07 December 1999).]
 
you may know this but the explanation for it's brittlness are over 2% carbon 14% chromium, and 5% vanadium.
 
I've got a Spyderco Starmate and they decided that they got the best strength and edge retention with 440V using only RC55/56. See the thread:

http://www.bladeforums.com/ubb/Forum20/HTML/001382.html

I found that the edge would roll when I used the Starmate to behead rabbits at the low rockwell. I believe that the blade wears better and is less likely to chip or break at 56RC. I prefer BG42 at around 60RC. I believe Tom Mayo hardens his 440V to a higher rockwell.

http://www.mayoknives.com/
 
The 440v and 420v perform better at 56 rc than at higher rc ratings . It is in the spec sheet. Before making a decission on the steel PLEASE have the blade tested for rc hardness..
420v outperforms most all other stainless steels I have seen and used if the heat treat is correct.
The problems start with bad process control..
and old habits of running the rc at the hardness required for ats and bg42.
Please find a qualified maker to build you a custom with the proper heat treat and I feel your mind will change.. and the steel will shine


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Web Site At www.infinet.com/~browzer/bldesmth.html
Take a look!!!




[This message has been edited by Darrel Ralph (edited 08 December 1999).]
 
Go back to the Crucible site and read the comparison graphs of 420V/440V with other steels, where it will be apparent why it's considered brittle.
 
dont know how 420v found its way into the discussion...it wasnt on the original post. the question is why- we can ask the same about the 308 bullet....the standard for military match...it doesnt stablize until around 80 yards....why...physics..thats why. its just the way it is....and i am one of those that feel 440V is on the brittle side, not so much that it is no good for a knife steel..but it needs more care than some other steels...from what ive seen 420v doesnt fall into this category.

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http://www.mayoknives.com


 
Darrel :

440v and 420v perform better at 56 rc than at higher rc ratings
Click to expand...

If they are softer they will indent more readily and roll much easier as Jeff noted in the above. At 55/56 RC they might be in the same impact toughness range of BG-42 at 60/61 RC, but the strength will not be anywhere close.

As for overall use, I have a 420V blade at 59 RC. So far I have seen no problems with very light use, it is a fillet blade and the steel works very well for that application. Storyville, if you are seriously interested in the CPM steels discuss the matter with Phil Wilson. He made the fillet blade and has been using the CPM grades for a long time, he also recently toured Crucible Labs.

-Cliff


[This message has been edited by Cliff Stamp (edited 08 December 1999).]
 
Thanks for the comments... I’m realizing that I should’ve titled this thread, “HOW brittle?”

What remains unclear to me is what 440V’s brittleness means in *practical* terms. As much of the statistical info loses me w/out “real world” examples to animate them, can someone translate its quantitative data into qualitative experiences? So we know it appears brittle on paper, but what kinds of real uses (particularly instances of failure?) illustrate the two-dimensional data represented by the bar graphs? For example, has anyone experienced ANY chipping? And if so, doing what?

Also, is Jeff’s experience w/his Starmate’s 55/56 RC edge *rolling* uncharacteristic? Cliff suggests no. If not, then I’m REALLY confused... At what point does it become “brittle,” and what does that mean for actual use? Is 440V’s brittleness possibly overstated or misunderstood?

Curioser and curioser --
Glen
 
Edge rolling is a matter not only of the steel but of the edge geometry. Actually I think the edge geometrry is more important than the steel. I too have a Starmate and it's an excellent knife. But it has a rather thin edge with a smaller angle than one would put on a larger fixed blade knife. If the edge was rolling, you probobly needed a heavier knife.

I used my Sebenza still with the factory edge to cut up an old credit card. After a few cuts I tried the edge with my fionger and it was rolled over real nice. Then I used one of my fixed blade knives of D2 at RC 58 to finish cutting the card up. It took substantially more force to cut the card, but the edge didn't roll. And although both knives were sharp enough at the start to shave hair, the Sebenza FELT much sharper due to it's fine edge. My fixed blade knife was sharpened with a 25 degree edge.

Point is, I don't think the result had anything to do with whether D2 is better than BG-42: it had to do with the edge geometry and using the right knife for the right job. IN the future I'll probobly just use scissors to cut up credit cards. :-)

BTW, it only took me a few minutes with a fine stone to bring the Sebenza edge back...

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www.wilkins-knives.com


 
Let me correct my post... FOR ALL AROUND USE IMHO.
I have been working with these steels- 420v ,440v and bg42 since they came out.
Phil Must feel its great material also..

Not all the answers are on paper or in spec sheets.
Sorry for the mispost with the 420v..


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Web Site At www.infinet.com/~browzer/bldesmth.html
Take a look!!!


[This message has been edited by Darrel Ralph (edited 08 December 1999).]

[This message has been edited by Darrel Ralph (edited 08 December 1999).]
 
dont want to step on anyones toes..but 44ov and 420v DONT HAVE ANY EDGE ROLL OVER..the stuff is hard as a rock and takes a super fine precise razor edge...no rollover...it chips (440v) before it rolls....from what i have seen bg42 doesnt have this problem either, but dont have enough feedback on it yet to say for sure.post script. on my hunting knives that i make out of these two steels...i grind the edge thin, and i grind a long bevel in when i sharpen it...and they dont roll over....they just cut. very well.

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http://www.mayoknives.com




[This message has been edited by tom mayo (edited 08 December 1999).]
 
Storyville, when people comment that CPM-440V/420V are brittle they mean relatively of course. If you took a CPM-440V and BG-42 blade at 61 RC the 440V one would be brittle in comparison. When you temper the CPM's down to the mid fifties it will obviously greatly improve their toughness. However it also greatly lowers the resistance to rolling and impacting, the latter being exactly what hardness measures and strength being strongly correlated to it.

-Cliff
 
Tom I agree with you hear . I have not had an edge roll from either440v or 420v.. I have had both chip out at higher rc than recommended from the manufacturer. This steel cuts better and longer than anything I have had In Average situations.. It takes more inch pounds to bend it also. It holds a fantastic edge..

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Web Site At www.infinet.com/~browzer/bldesmth.html
Take a look!!!




 
storyville,

A 440V blade heat treated right most likely will not chip. It is more brittle than some of the other high end steels, which is why it requires a lower rockwell. The lower rockwell counteracts the brittleness. But by lowering the rockwell you have more denting and rolling. Also with the lower hardness your edge geometry is more limited.

If your blade geometry is pretty ordinary, and if your edge angle is not too thin, then the softness issue is not that important, except when you cut harder things. Normal everyday cutting your 440V's edge should last longer than most of the other steels. However, if you hit that occasional staple, or cut harder things; the steels that can be hardened harder would do better.

Hope this is clear.

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Johnny
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to all interested parties....440Vs recommended Rc is at 56-58....I asked paul bos to lower the rockwell so we could increase the toughness after i began to see how brittle 440V really was...the response. under 56 Rc the corrosion resistance goes OUT the window...life is a balance..choose the things you put on each side carefully. from everything i have ever learned..we only go around once.

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http://www.mayoknives.com


 
OK (deep breath) -- please follow my not-very-sophisticated thinking here; I apologize for my continuing stubbornness! I appreciate the wide interest, but some replies here are ignoring rather than responding to my questions. Once again, I have reviewed the stats, bar graphs, and related info available on Crucible’s site and other sources, so I AM AWARE OF ITS STATED PROPERTIES.

However, some users of 440V have reported behaviors which appear INCONSISTENT w/its reputed characteristics. I started this thread because I am puzzled by an apparent discrepancy between *SOME* REAL WORLD uses and ON-PAPER expectations, particularly regarding 440V’s reputed brittleness relative to other steels. SPECIFICALLY:

1) Outlaw_Dogboy’s 440V Spyderco Native experienced notable DENTING when it hit a magazine staple. Its hardness was measured by Spyderco after-the-fact at 55.5 RC.

2) Similarly, Jeff Clark (near the top of this thread) found that his 55/56 RC Starmate would ROLL while beheading rabbits.

3) Additionally, I recently learned of the work of Steve Mullin, who offers small outdoor knives in 440V. Paul Bos apparently treats these 440V knives to 59-60 RC. In an e-mail I just received, Steve Mullin states that he has received no reports of chipping from normal use (e.g., dressing game, whittling wood). In fact, in the past couple of years he has received only one report of “some microchipping” (customer’s words) from a customer who was digging dirt (!).

So here’s what confuses me: If we take these three reports at face value (I have no reason to doubt them), they appear to contradict “on-paper” characteristics of 440V, no? Spyderco is a well-respected company, but perhaps its production heat-treating is less careful than that given to handmade knives? On the other end, Steve Mullin appears to have no complaints, to date, of brittleness at 59-60 RC. But I also fully respect and believe Tom Mayo and Darrell Ralph’s statements that their knives treated to 56-58 RC maintain excellent, lasting, wear-resistant edges which essentially perform to specs, and that they’ve seen chipping at higher RC.

What accounts for these widely varying results? I am NOT asking, “how is 440V SUPPOSED to behave?” Rather, “WHY has it behaved in these ways”?

Still stubbornly perplexed,
Glen
 
i am trying to answer your question..i did say at the top...the answer is physics. i would compare 440V to glass....because of the addition of the vanadium...it forms all these great molecules that are hard...it cuts well..but like glass is brittle...it has its drawbacks...i like what steve mullin had to say-microchips..thats what ive seen.. but still...chips they are...only at the very edge....you can sharpen the knife aggresively and they are gone...but they were there...and that makes me say its brittle...why...because all the stuff in the steel goes into making a molecular structure that acts like that....i cant go much deeper than that....sorry.

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http://www.mayoknives.com


 
I think that Spyderco heat treatment is as well controlled as many custom knife manufacturers. If they are trying for a tightly controlled rockwell of between 55 and 56 and when they test a sample it is 55.5 they are doing excellently! They have their own priorities for blade performance for their knives. It looks to me like they want none of their premium knives to fail by chipping or having the point break. They also have a priority to have the knives stay sharp a long time in common usage. They best achieve these broad goals using the specific heat treat and hardness of 55.5 Rc. Edge rolling is not generally a big deal. You can fix it with a smooth steel and a ceramic rod in just a few strokes. I just don't want to carry the steel with me when I'm hunting.

A custom knife maker is likely to make a more specialized type of knife with a little more beef to the blade than in a folder. They may pick different priorities and tailor blade design and hardness for a different optimum.

You'll notice that Tom Mayo has been adjusting his process around. He made harder 440V blades in the past and never saw edge rolling. Maybe we could get a little more info out of him about how he decided to change. Maybe it was just bad word of mouth.
 
There are at least two working definitions of 'brittle'. One would be in comparison to other steels, like Crucible has done in comparing 440V to their other steels, a comparison that no amount of personal testimony will change, and the other relative to expected use. An edge rolling suggests a ductile condition while one chipping suggests what some would call brittle. Edge geometry can minimize problems but can't make them go away, which is the see-saw balance that knife makers and knife users have to make when they decide between different properties. I don't think that the 'brittleness' is a problem, provided one knows of the condition and is willing to consider the consequences of hard use, as the edge holding appears to be excellent.
 
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